In radio communications systems, information (for example voice, image information or other data) is transmitted by electromagnetic waves via a radio interface between a transmitting and a receiving station (base station and subscriber station, respectively). The electromagnetic waves are in this case transmitted at carrier frequencies which are in the frequency band intended for the respective system. Frequencies in the frequency band around 2000 MHz have been provided for future mobile radio systems using CMDA or TD/CDMA transmission methods via the radio interface, for example the UMTS (Universal Mobile Telecommunication System) or other 3rd generation systems.
Transmission circuits in radio communications systems generally have a limited dynamic range in which they operate or work efficiently with regard to the power. For a number of applications, in particular in mobile radio, the distance between the transmitter and the receiver often fluctuate to a major extent during operation. It therefore becomes desirable to adapt the transmission power level over a range of up to several orders of magnitude in order to keep the ratio of the energy per bit to the noise power density (Eb/No) or the ratio of the signal to interferer or carrier power to interference power (C/I) within the limit range or desired range. Firstly, the received power must have a minimum level which is required for the desired Quality of Service (QoS), but, secondly, as little interference as possible should be produced.
Apart from the distance, the data rate may also be variable. For a constant Eb/No the mean transmission power level must therefore be adapted not only in response to any change in the path loss, but also in response to any change in the data rate.
In systems which use a spread band technique, the spread factor also varies with the data rate. The lower the data rate, the higher the process gain and the lower the mean transmission power level must be for a constant Eb. Low data rates occur, for example, during voice transmission, when only a silent description is transmitted during pauses in speech.
The power consumption of a transmission circuit is in this case generally not proportional to the transmission power level. The lower the transmission power level is chosen to be, the poorer is the efficiency. Particularly in mobile radio systems, this represents a problem since, firstly, there is an aim to reduce the transmission power level of mobile transmission and receiving stations (MS) to an even greater extent, which leads to the equipment having a relatively high power consumption. Secondly, these mobile stations are intended to be ever smaller, lighter and more convenient. As the sizes reduce, less and less space is available for batteries or rechargeable batteries. That is, the capacity is comparatively low. Both factors lead to a reduction in the standby time and transmission time. Although the rechargeable batteries have also been developed further in parallel with the reduction in size of the mobile stations, the gain in capacity is comparatively low in relation to the reductions in capacity mentioned above.
Various approaches are known for solving the problem mentioned above.
When using the spread band technique, a low power level is normally used for transmission, with a low path loss and a low data rate (that is to say a high spread factor). However, reducing the transmitter power results in the transmission circuit having poor efficiency.
In the multislot technique, the data rate is increased or decreased by making use of a greater or lesser number of timeslots. The duty cycle of the transmitter is reduced when the data rate is reduced, and hence the mean transmission power level. However, this does not reduce the efficiency, or reduces it to a lesser extent than when transmitting with the same duty cycle and a lower amplitude. This technique is known from the “Enhanced Slotted DL transmission mode”, ETSI SMG2 Layer 1 Expert Group Meeting, April 1998, Oslo, Norway.
In a method with power control, the transmitter is intended to be able to reduce its transmission power level to a fraction of the maximum power—for example by around 30 dB for GSM (Global System for Mobile Communication), and by around 80 dB for UMTS. One disadvantage in this case is that the power consumption of the transmission circuit is reduced to a considerably lesser extent than the power output. Particularly in battery-operated transmitters, this is unsatisfactory since the transmission time and speech time are extended to only a very limited extent by reducing the transmission power level. Such a technique is known from the “Mobile station TX power”, ETSI SMG2 UMTS L1 Expert Group Meeting, September 1998, Helsinki, Finland.
In other words, power control is in this case replaced by Eb control (Eb: Energy per coded bit) with the latter being possible not just by control of the transmission power level, but also by control of the channel coding rate.